This invention relates generally to glass-ceramic cooktop appliances and more particularly to improving temperature measurement therein.
The use of glass-ceramic plates as cooktops in cooking appliances is well known. Such glass-ceramic cooktops have a smooth surface that presents a pleasing appearance and is easily cleaned in that the smooth, continuous surface prevents spillovers from falling onto the heating unit underneath the cooktop.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from a heating unit, such as an electric coil or a gas burner, disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the heating unit to heat utensils upon it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses a heating unit that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooktop is heated primarily by radiation transmitted directly from the heating unit to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktops are more thermally efficient than other glass-ceramic cooktops and have the further advantage of responding more quickly to changes in the power level applied to the heating unit.
In both types of glass-ceramic cooktop appliances, provision must be made to avoid overheating the cooktop. For most glass-ceramic materials, the operating temperature should not exceed 700° C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with no load, i.e., no utensil, on the cooktop surface, using badly warped utensils that make uneven contact with the cooktop surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic from extreme temperatures, glass-ceramic cooktop appliances ordinarily have some sort of temperature sensing device that can cause the heating unit to be shut down if high temperatures are detected. In addition to providing thermal protection, such temperature sensors can be used to provide temperature-based control of the cooking surface and to provide a hot surface indication, such as a warning light, after a burner has been turned off.
One common approach to sensing temperature in glass-ceramic cooktop appliances is to place a temperature sensor directly on the underside of the glass-ceramic plate. With this approach, however, the temperature sensor is subject to the high burner temperatures and thus more susceptible to failure. Moreover, direct contact sensors detect some average flux and do not produce a direct measurement of the glass-ceramic temperature. Thus, it is desirable to use an optical sensor assembly that “looks” at the glass-ceramic surface from a remote location to detect the temperature of the surface.
A remote sensor assembly determines the glass-ceramic temperature based on the amount of radiated flux it receives from the glass-ceramic plate or a utensil. However, in addition to flux radiated from the glass-ceramic plate, the sensor will receive flux that is reflected from the bottom of the glass-ceramic plate. This is because during operation of the cooktop appliance, the heating unit emits energy that strikes the underside of the glass-ceramic plate. Some of this energy will be absorbed by the glass-ceramic plate (thereby raising the plate temperature), and some of the energy will be transmitted through the glass-ceramic plate. The rest of the energy, which is not an insignificant amount, will be reflected by the glass-ceramic plate. The reflected flux that strikes the sensor assembly will affect the accuracy of the temperature measurement. Flux reflected from other sources, such as metal burner components or a utensil placed on the glass-ceramic plate, can also strike the sensor assembly. The sensor assembly can also receive flux from background radiation sources, such as ambient lighting, that is transmitted through the glass-ceramic plate.
Accordingly, there is a need for a remote sensor assembly that can reduce and/or compensate for corruptive flux.